API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

2-128 API RECOMMENDED PRACTICE 581
19 High Temperature Hydrogen Attack (HTHA) DF
19.1 Scope
The DF calculation for carbon steel, C-½ Mo, and Cr-Mo low alloy steel components subject to HTHA is
covered in this section.
19.2 Description of Damage
HTHA occurs in carbon steel, C-½ Mo, and Cr-Mo low alloy steels exposed to a high partial pressure of
hydrogen at elevated temperatures. It is the result of atomic hydrogen diffusing through the steel and
reacting with carbides in the microstructure. There are two reactions associated with HTHA. First the
hydrogen molecule, H 2 , must dissociate to form atomic hydrogen, H, which can diffuse through steel.
H2 ⇔ 2 H
(hydrogen dissociation)
The reaction to form atomic hydrogen occurs more readily at higher temperatures and higher hydrogen
partial pressures. As a result, as both the temperature and hydrogen partial pressure are increased, the
driving force for HTHA increases. The second reaction that occurs is between atomic hydrogen and the
metal carbides.
4H + MC ⇔ CH 4 + M
Damage due to the HTHA can possess two forms:
1) internal decarburization and fissuring from the accumulation of methane gas at the carbide matrix
interface;
2) surface decarburization from the reaction of the atomic hydrogen with carbides at or near the surface
where the methane gas can escape without causing fissures.
Internal fissuring is more typically observed in carbon steel, C-½ Mo steels, and in Cr-Mo low alloy steels at
higher hydrogen partial pressures, while surface decarburization is more commonly observed in Cr-Mo steels
at higher temperatures and lower hydrogen partial pressures.
HTHA can be mitigated by increasing the alloy content of the steel and thereby increasing the stability of the
carbides in the presence of hydrogen. As a result, carbon steel that only contains Fe 3 C carbides has
significantly less HTHA resistance than any of the Cr-Mo low alloy steels that contain Cr and Mo carbides
that are more stable and resistant to HTHA.
Historically, HTHA resistance has been predicted based on industry experience that has been plotted on a
series of curves for carbon steel and Cr-Mo low alloy steels showing the temperature and hydrogen partial
pressure regime in which these steels have been successfully used without damage due to HTHA. These
curves, which are commonly referred to as the Nelson curves, are maintained based on industry experience
in API 941.
19.3 Current Status of HTHA Investigations and Inspection
In 2010, an incident within the refining industry led to an investigation where HTHA was identified as the
damage mechanism that led to the failure of a heat exchanger. The refining industry has been examining the
findings published in the Chemical & Safety Board report, along with new information from the industry
concerning HTHA damage.